In digital, high-speed serial communication systems, binary signals are transmitted between a transmitter and a receiver as a series of square pulses, which are used as basic communication symbols. Characteristics of the transmission channel, such as the type of metal used and the type of connectors, distort the signals. One type of distortion, referred to as intersymbol interference, occurs when the transmission medium propagates the frequency components of the data (symbols) at different rates, causing the symbols to overlap from one bit interval to another.
FIG. 1 is a diagram showing a signal transmitted over a channel having bit intervals of time “T”. A single pulse transmitted at time 0 is passed through the channel. As shown, intersymbol interference caused by the channel results in a significant spillover of the bit-interval's energy into the succeeding bit that was transmitted between time T and 2T. One well-known method for reducing such intersymbol interference is through the use of an equalizer. The type of equalizer being used herein is basically a filter having an approximate inverse transfer function to that of the channel (transmission medium) that is used to correct amplitude and delay distortion. This is accomplished by de-emphasizing energy in the proper frequency band. Through the use of this equalizer, the transfer function associated with frequencies within the passband can be made relatively flat, which reduces the potential for one symbol to interfere with another in the time domain.
One type of equalizer includes current-mode differential drive circuits that are controlled by a finite impulse response (FIR) filter. In its general form, the filter comprises a combination of a shift register containing the current outgoing data bit, and a history of the three previous data bits. After a drive circuit outputs a signal corresponding to the first bit, the signals for the subsequent bits are altered on each bit boundary to compensate for the characteristics of the transmission medium, such as a serial link. Typically, the signal for each bit is altered by reducing its amplitude to address the distortion.
Although performing equalization with an FIR that uses bit boundary switching reduces intersymbol interference, one disadvantage is that altering the signals only at the beginning of each bit interval can slow the response of the filter and fails to maximize channel response. Accordingly, a new and improved equalization scheme is desired. The present invention addresses this need.